1. Field of the Invention
The present invention relates to a method for manufacturing a piezoelectric resonator in which vibrating electrodes are disposed on both surfaces of a piezoelectric substrate and piezoelectric vibration is trapped between the vibrating electrodes, and more particularly, the present invention relates to a method for adjusting the frequency of the piezoelectric resonator.
2. Description of the Related Art
In recent years, in the field of piezoelectric resonators used in, for example, oscillators or filters, there is an increasing demand for accurately adjusting the frequency. Previously, ink is applied onto an electrode, and the frequency is adjusted by a mass load of the electrode. In this method, however, variations in the application quantity of ink are large, and the concentration of the frequency is low, thereby failing to adjust the frequency accurately.
To solve this problem, Japanese Unexamined Patent Application Publication No. 5-29864 discloses a frequency adjusting method in which vibrating electrodes are formed on both surfaces of a piezoelectric substrate, and a metallic film, for example, a metallic thin film, having a predetermined thickness is formed on each vibration electrode by a dry process.
More specifically, vibrating electrodes having a desired final configuration are formed on a piezoelectric substrate. Then, etching resist ink is applied onto the overall surface of the piezoelectric substrate, except for the portion on which the vibrating electrodes are disposed, and a metallic material is attached onto the vibrating electrodes by vapor deposition or sputtering. Then, the etching resist ink is removed. Instead of using an etching resist ink, the overall piezoelectric substrate may be covered with a metallic mask, except for the portion on which the vibrating electrodes are formed, and a metallic material is attached onto the vibrating electrodes by vapor deposition or sputtering.
Japanese Unexamined Patent Application Publication No. 6-224677 discloses the following technique. A metallic film is formed on each surface of a piezoelectric substrate, and the resulting piezoelectric substrate is then polarized. The frequency is then measured, and a vapor deposited film is formed until a target frequency is reached and is then patterned into the configuration of an electrode.
In the method for forming a metallic film on the vibrating electrodes by a dry process, such as that in Japanese Unexamined Patent Application Publication No. 5-29864, the thickness of the film can be precisely controlled, and thus, the resonant frequency can be adjusted accurately in very small increment/decrement units of ±0.1%.
In this method, it is necessary to overlay a metallic film only on the vibrating electrodes. However, in either of the method for using etching resist ink or a metallic mask, it is difficult to accurately position the etching resist ink or the metallic mask to the vibrating electrodes, thereby making the frequency adjustment difficult. In particular, recently, as the size of piezoelectric resonators becomes smaller, the structure of vibrating electrodes becomes finer. In this case, a slight deviation of etching resist ink or a metallic mask seriously influences the adjustment precision.
FIGS. 13A and 13B illustrate the conditions in which a metallic thin film 61 is formed on a vibrating electrode 60 on a piezoelectric substrate 62 by a known method. More specifically, FIG. 13A illustrates the condition in which the metallic thin film 61 is accurately formed on the vibrating electrode 60, and FIG. 13B illustrates the condition in which there is a deviation 6 between the vibrating electrode 60 and the metallic thin film 61.
If the metallic thin film 61 is accurately formed on the vibrating electrode 60, as shown in FIG. 13A, the thickness of the resulting vibrating electrode is uniform. In contrast, if there is a deviation 6 of the metallic thin film 61 from the vibrating electrode 60, as shown in FIG. 13B, the resulting vibrating electrode has a thicker portion and a thinner portion, i.e., the thickness of the resulting vibrating electrode is not uniform, thereby making it difficult to adjust the frequency to be a target frequency.
The adjusting method disclosed in Japanese Unexamined Patent Application Publication No. 6-224677 presents the following problems since the first formation of a film for electrodes is performed on the overall surface of the piezoelectric substrate.
One problem is that trapping of piezoelectric vibration becomes insufficient when measuring the frequency, and thus, a spurious response is generated, thereby making it difficult to accurately measure the frequency.
The other problem is that, since the film deposition area is large, the temperature of the piezoelectric substrate becomes higher if film deposition is performed by sputtering, thereby encouraging the depolarization. In other words, the piezoelectric substrate may be depoled.